1. Field of the Invention
The present invention relates to the field of antenna structures for portable apparatus applications and more specifically to compact or miniature antenna equipment suited for use with miniaturized portable radio communication apparatus.
2. Description of the Conventional Art
FIG. 14 shows a sectional portable radio communication apparatus equipped with a conventional helical antenna. Referring to the figure, an insulating resin housing 101 houses a radio circuit 103 enclosed in a metal frame 102. A helical antenna 104 is covered by an insulating cover 106 and fixed perpendicularly on a top surface of the resin housing 101. The helical antenna 104 of helically wound line conductor has an approximately .lambda./4 wavelength and is connected directly to the radio circuit 103 via a connection lead 105. An impedance matching circuit is not required in this case for the low load impedance of the antenna 104 because the voltage of a received wave is low enough at the feeder end connected to the connection lead 105 to match the radio circuit 103.
The approximately .lambda./4 wavelength helical antenna 104 acts as an approximately .lambda./4 wavelength monopole antenna when fed by the radio circuit 103. The metal frame 102 having an approximately .lambda./4 wavelength serves as a ground plane for the helical antenna 104 when insulated from the operator's hand or body by the resin housing 101. The combination of the approximately .lambda./4 wavelength helical antenna 104 and the approximately .lambda./4 wavelength metal frame 102 achieves an antenna performance or radiation pattern corresponding to that of a half-wave dipole antenna.
FIG. 15A shows a horizontal plane radiation pattern of a dipole antenna according to the conventional helical antenna of FIG. 14 and FIG. 15B shows the vertical plane radiation pattern as actually measured.
FIG. 16 shows a VSWR, Voltage Standing Wave Ratio, characteristic of the conventional helical antenna 104.
Thus, the conventional approximately .lambda./4 wavelength helical antenna acting as a monopole antenna effects the miniaturization of the antenna. The conventional approximately .lambda./4 wavelength helical antenna is physically smaller, for example, than a .lambda./4 whip antenna in physical length and is suited to a miniature portable radio communication apparatus.
The conventional helical antenna of the helically wound line conductor, however, is easily affected by surrounding conductors. Coiling or winding of line element can result in deviations in size and in dimensions lacking precision, thereby resulting in failure to provide consistent antenna performance. The conventional helical antenna is also limited as to the degree of miniaturization. Helically wound line elements have physical limits of miniaturization which make them inapplicable to highly miniaturized portable radio communication apparatus such as pagers and wristwatch-type radio communication apparatus.
The present invention is directed to solving the foregoing and other problems by providing antenna equipment suited to miniaturized portable applications where the antenna must be compact enough for the highly miniaturized housing of portable radio communication apparatus and wherein the effects of surrounding conductors such as the user's body are eliminated by detuning.